A similar question was asked before, and this comment by u/integral_grail is pretty thorough. The entire thread is pretty interesting to peruse on this topic: https://www.reddit.com/r/AskHistorians/comments/7kttyy/ayatollah_ruhollah_khomeini_issued_a_fatwa/drh4iud/?utm_source=share&utm_medium=ios_app&utm_name=iossmf&context=3